DEVELOPMENT OF ELASMOBEANCH FISHES. 21 



appearance of striation may on this hypothesis be explained 

 as due to the presence of granules in the protoplasm. When 

 the streaming out of the protoplasm of a nucleus into that of 

 a cell takes place, any large granule which cannot be moved by 

 the stream will leave behind it a slack area where there is no 

 movement of the fluid. Any granules which are carried into 

 this area will remain there, and by the continuation of a pro- 

 cess of this kind a row of granules may be formed, and a series 

 of such rows would produce an appearance of striation. In 

 many cases, e.g. Anodon, vide Flemming^, even the larger yolk- 

 spherules are arranged in this fashion. 



On the supposition that the striation of these figures is 

 due to the outflow from the nucleus, the appearances presented 

 in Elasmobranchs admit of the following explanation. 



The central body consisting of two cones (Figs. 7a, 7c) is almost 

 without question the remnant of the primitive nucleus. This 

 is shewn by its occupying the same position as the primitive 

 nucleus, staining in the same way, and by there being a series 

 of insensible gradations between it and a typical nucleus. The 

 contents must be supposed to be streaming out from the two 

 apices of the cones, as appears from the striae in the body con- 

 verging on each side towards the apex, and then diverging 

 again from it. In my specimens the yolk-spherules are not 

 arranged with any reference to the radiating striation. 



It is very likely that in the cases of the disappearance 

 of the nucleus, its protoplasm streams out in two directions, 

 towards the two parts of the cell which will eventually become 

 separated from each other; and probably, after the division, the 

 matter of the old nucleus is again collected to form two fresh 

 nuclei. 



In some cases of cell-division a remnant of the old nucleus is 

 stated to be visible after the fresh nuclei have appeared. These 

 cases, of which I have not seen full accounts, are perhaps 

 analogous to what occasionally happens with the germinal 

 vesicle of an ovum. The whole of the contents of the germinal 

 vesicle become at its disappearance mingled with the proto- 

 plasm of the ovum, but the resistent membrane remains and 

 is eventually ejected from the Qgg, vide p. 3 et seq. If the 



1 Loc. cit. 



